Add forceSingleColumn prop to <UI /> (#10807)

* Move TabsBar rendering logic from CSS to the ColumnsArea component

* Add forceSingleColumn mode

* Add unread notifications counter to tabs bar

* Add toggle to control `forceSingleColumn`

* Increase paddings in mobile layout responsively at large sizes

app/javascript/mastodon/components/autosuggest_input.js

@@ -49,7 +49,7 @@ export default class AutosuggestInput extends ImmutablePureComponent {
     autoFocus: PropTypes.bool,
     className: PropTypes.string,
     id: PropTypes.string,
-    searchTokens: PropTypes.list,
+    searchTokens: ImmutablePropTypes.list,
     maxLength: PropTypes.number,
   };